Beat frequency navigation and guidance system



June 22; 1954 Filed Dec. 14, 1949 swzsp FREQUENCY Gamma-r0? P3085 RECE/l/E'R S. R- RICH BEAT FREQUENCY NAVIGATION AND GUIDANCE SYSTEM 2/ 2 TIMA'SMITI'ER v FEEQUEIVCY 3 Sheets-Sheet 1 29 ens/V501 FREQUENCY DISC/P/M/AATl/VG CUMPHRA UR M 5mm:

DIRECT/0N CORRECT/ON SER V0 3 Sheets Sheet 2 5. R. RICH 0 V Q 6 8 6 G 6 1| H 1 F E 7 9 7 w fi A 6 a J u v m n 0 n 6 c m 6 3 4 5 5 m 4 4 4 J. J J m 7/1 5 w! w v J F 4/ l llrll L GEHS $55. 5, mam M m 7 r 4 4 a m o 5 w W n kw M June 22, 1954 BEAT FREQUENCY NAVIGATION AND GUIDANCE SYSTEM Filed Dec. 14, 1949 SWEEP Fl? FREQUENCY DISCRIMINAT/NG and C OMPA PING C ZSZCUI T L l5 i l i a 77MECVCL/NG 7/ sums/2512 ASVMMETPIC L/M/TEI? L/MITER AMPLITUDE AMPLITUDf Flam RECEIVE/2 F2044 RECEIVER June 22, 1954 5 c 2,682,049

BEAT FREQUENCY NAVIGATION AND GUIDANCE SYSTEM Filed Dec. 14, 1949 3 Sheets-Sheet 5 4o i 37 /2 w2'2/% 5 ,1 ENE/261 ZER I20 L AMPL/i'UDE swssg. /4-

H6 Ma0u AT0/2 MODULAQO/Z /N VEN TOR Patented June 22, 1954 BEAT FREQUENCY NAVIGATION AND GUIDANCE SYSTEM smiley 1i. Rich, Newton Center, Mass,

Ray'theon Manufact Mass; acorrelation assigno'r uring' Goiiipan'y; Newtmi; f Delaware Application December 14, 19 19, eriail No. 132,382

3 Cia'ims. I

This" ifiVfitidfi relates t6 fiaiiiatidn afid Quidaiie svstem's; and more particularly to beat freuency navigation aiid" gil'l'dafi systems.

Heretofore, because of the limitations inherent in pulse systems and also in' existing low frequency continuous wave systems,- the two transmitters of a given hyperbdlic' pair were required to be separated many miles apart in order to provide usef-ul position resolution. In the present invention, however, by using a beat f'requency technique, it becomes possible to decrease this base line distance to a few yards. it also becomes possible to dispense with one of the transmitters formerly required and use a singletransmitter as the signal source for two signal radiators or antennas. The present invention also permitsa simplification oi the receiving apparatus for use withsuch a beat-frequency system andmakes possible very simple means for detecting deviation from a given desired course.

When two similar sweep-frequency signals, having a linear saw-tooth sweep over a band of frequencies one lagging the other in tithe, are radiated from separated points a horizontalplane, a hyperbolic grid work ofdifferent beat-frequency lines may he traded by a receiver in thearea covered by the si'gnals. The signal radiations, from the radiators as foci} produce these hyperbolic lineswhich are the loci of particular beat frequencies caused by the mixing of the radiated signals along each of the lines. The beat frequency of each such hyperholic line isdi'fierent and the pattern of these lines defines a frequency spectrum determinedby the instantanedus di'iie'r'ence in frequency between the radiated signals and the distance between the sigiial' radiator-s. The frequency of succeeding lines", as one passes from one radiator to the other; progressively'ii'ici'eases from the lower freuency radiator to the higher oil; the equilateral hyperbole, which is" a} straight line running between the fa diatZirS mini-distant fr m each radiator and perpendicular to theline described theiii', fl'i beat' frequency is equal 6 0- the ihstatneeus' diffefe'he in frequen ie of Too'ne' side of this line, beat freque ciesprog' jssivew' ncrease whee, to tlie'other side of thisli e", beat frequencies pror'essiveiy decrease. Tim's, when'uie' beat fre- .4 of thisline is" Known, a lieafi-flfdfie hcy sensitive sody'mnving in this; region will have a left' and right sense of' direction with respect to this line as a reference.

In like manner, another similar set of signal radiators in a vertical plane gives an up and a 2 down sense of direction. When the two sets of sig radiators are arranged perpendicular to an I ectin'g each other, so that the equilateral hyperbole of one of them is made to coincide with the equilateral hyperbola' of the other, the coincident path be'cor'ri'es' thereby one with a left and right and up'and' down sense of directions. Such a desirable pattern becomes useful for guiding moving objects along the coincident path either toward a distant tar'get or for homing purposes.

Other hyperbolic paths in the above-described gri'dwork' may also be used to perform important functions; For example, the beat frequency of a hyperbolic line suitable for providing the landing path of an airplane may be selected" from the pattern of one se't'of radiators placed along the runway of a landing strip. Because of the increasing and decreasing frequency eiiects above and below this reference path, as already explained, an up and down sense with respect to this path as a reference is provided. The left and rightseiis'e may likev'rise be provided by a horizontally placed" set of A radiators perpendicular to the line described by the first-mentioned set.

In the present embodiment of this invention, suchanovel gu'ida' nc'e gridwo'rk is achieved by providing gefier'ally two radiators at separated points it l'1'=a-s'igiial link connecting their'L- A linear sweep n-equency generator is linked to one of the radiators so that; because of a signal travel distance between the two radiators; the signal from the generator is emit-ted from one radiator a short time'before it reaches-the other for emission-.- This timelag introduced by the distance between the radiators; results in a different frequency being emittedfrom each transmitter at any given instant. By using a saw-tooth sweep at the common ghal souree, this spread of transmitted frequenciesrein'ains the same throughout most of each cycle. As a practical result, a stable hyperbolic" gridw'ork of the type" explained above is" created;

one embodiment of the present invention'incorporates the stove in a guidance system which operates by comparing to a reference beat frequency of the" same frequency as that of a se l-ec'ted hyperbolic path in the gridwork the beatfrequency determined by the position ofthe sig-" nal sensitive body. The results of such a com parison may be registered ona suitable meter showing degree and direction of deviation from the referenceh'yperbolav other applications, this comparison may be used to operate, for example, a servo device controlling the movement of the body, to thereby direct its course on the path of the selected hyperbola.

To insure a proper reference beat frequency in the signal sensitive moving body, the beat frequency of a desired hyperbola is obtained by a probe located at a convenient point on the selected hyperbola and linked to a beat-frequency detecting receiver. The beat frequency so detested is then retransmitted by a carrier of a different frequency from any of those emitted by the pairs of radiators.

In another embodiment of this invention for navigation or guidance along a selected hyperbolic path, an alternative system for giving a left and right sense of error with respect to the given reference is achieved. By alternately switching the signal of the frequency modulated generator from one radiator to the other, the time lag is periodically reversed between the radiators. This causes a reversal of the hyperbolic gridwork of beat frequencies. The lines which designated the low frequencies are changed to lines designating high beat frequencies and. vice versa. Only the selected hyperbola bet veen the radiators continues to have an unchanging beat frequency. Further by producing these reversals in accordance with a time sharing plan, such as a cyclic operation with three units of time to one radiator, and one unit of time to the other radiator, a left and right Sense as to the selected hyperbola is produced. On one side of this hyperbola, one beat frequency registers for three units of time and another beat frequency registers for only one unit of time, while on the other side of this line the time condition is reversed.

The above is an illustration of time sharing asymmetry. In the system just described, the sense of direction of rror is provided by identifying the higher or lower beat frequency with the length of time of its recurrence in each cycle. The case of three units to one cited above is but an example. The times in the asymmetric cycle may have any ratio as long as they are not equal.

This alternate system is achieved by providing two signal links between the antennas with an asymmetrical time cyclin device operating a double throw switch for alternately sending the generated signal first to one antenna, and then to the other antenna, and simultaneously altering the length of signal link between the antenhas, thereby providing reversals in the hyperbolic gridwork pattern about a selected hyperbola.

In a third embodiment of this invention, the above hyperbolic gridwork is made to alternate with an amplitude modulated fixed frequency reference signal in a cyclic time sharing plan. The rate of amplitude modulation of the fixed frequency reference signal is at the same frequency as the beat frequency of the selected hyperbola in the gridwork.

Apparatus for operating in the gridwork patterns generated as mentioned above consists generally of suitable antenna and receiver apparatus arranged to receive and detect the beatfrequency signals at a point in the gridwork corresponding to the position of a moving body in this gridwork, and suitable frequency discriminating and comparing circuits for these received signals.

The foregoing and other advantages, objects and features of the invention will be better understood from the following description of exemplifications thereof, reference being had to the accompanying drawings forming part of this specification, wherein:

Fig. 1 is a schematic view of a preferred embodiment of the invention;

Fig. 2 shows a typical frequency versus time graph for the output of a sweep-frequency generator in Fig. 1;

Fig. 3 is a view of a hyperbolic gridwork pattern produced in the invention;

Fig. 4 is a schematic view of a second embodiment of the invention;

Fig. 5 is a beat-frequency measuring diagram for determining the position of a selected hyperbola used in the invention;

Fig. 6 is a graph of output voltage versus beat frequency of atypical frequency discriminating circuit in Fig. 4;

Fig. 7 is a graph of typical voltage versus time in the comparator circuit shown in Fig. 4;

Fig. 8 is a schematic view of a frequency discriminating comparator suitable for use in Fig. 1;

Fig. 9 is a schematic view of a sweep-frequency generator and antenna system suitable for use in the invention;

Fig. 10 is a view of a suitable cyclic timing switch energizer for use in the invention; and

Fig. 11 is a schematic view of a third, embodiment of a signal generating apparatus in the invention.

Referring to the drawings in more detail, Fig. 1 illustrates an embodiment of this invention particularly adapted to operate on a selected beatfrequency hyperbola by a comparison of the beat frequency of the selected hyperbola to the beat frequency determined by the position of a receiver in a hyperbolic grid-work pattern. Varying frequency signals, which, for example, vary in the range from 900 to 1000 megacycles, are generated by sweep-frequency generator l0. ()ne such generator suitable for use in this invention to be described hereinafter is shown in Fig. 9. The signal frequency of generator 10 is made to vary linearly with time as illustrated in Fig. 2. The signals so generated are sent over lines I! and i2 to directional antennas i3 and Ill, in this instance directed along parallel lines as shown.

onds sooner than antenna Hi.

tenna iii. Between antennas Because of the distance between antennas l3 and M, which in this instance is approximately ft., the signals generated by the generator it will reach antenna 53 about 0.l02 10- sec- Thus, as illustrated in Fig. 2, at a given instant of time, the frequency i3 emitted from antenna 13 is, in the present embodiment, about 11,450 cycles higher than the frequency 14 emitted from an- !3 and Hi may be thereby traced a hyperbolic gridwork of constant beat-frequency lines varyin from 1 to 22,900 cycles/seconds, some of which are illustrated in Fig. 3 as extending from the antenna base line due to the directional nature of antennas l3 and M, Non-directional antennas would produce a similar pattern on both sides of the base line.

It should be understood that the same result may be obtained by separate generators for each of antennas l3 and 14, provided that they are both sweep-frequency generators synchronized to sweep at the same rate over the same interval of time. The use of a single sweep-frequency generator in the present embodiment is made possible by the nature of the beat-frequency system and simplifies the problem of synchronizing the rate and period of sweep at the separate antennas.

To obtain a reference frequency for beat-frequency comparison the present. embodiment has.

generally a probe; ta which may be asuitable. antenna. located at; a convenient. potmt; on the particular hyperbol-a. selected for use in this system.. The signals: picked up byprobe. I are sent by lineit: to a. conventional: amplitude 1110611118,? tion. detecting receiver it; whose: radio frequency tuning is broad enough to. cover: al1 of the frequencies swept by: the; sweep izrequcncy genera.- tor; and: whose audio: circuits amplify beat frequency only. IThe: beat. fre uency of the: selected. hyperbola is: detected; and amplified: by receiver ill and sent asa: beat-frequency signal byline E8 toa conventional amplitude: or; frequency; modulated transmitter 245;. where it is made to. modulate a suitable carriersignal which isthensent. by line:- 21' to an antenna 27.: for radiae tion. The frequency of the; carrier. signal lies outside. of the range of frequencies used tO-prodose the hyperbolic glidWOhk. pattern: so; as to preventinterference;

A suitable; receiver; apparatus for navigation or guidance.- in the above-deescribed hyperbolic gridwori: pattern is; shown inv the right-hand side of Fig. 1. This apparatus is located on a body (not shown) navigating or being. guided within the gridwork. iwo antennas: 213i and-.2 are led by lines 25 and. 216,. respectively, to receivers 2i and28. Receiver 27 isa conventional amplitude: modulation receiver tuned. to" receive the reference. frequency signal radiated from antenna 22 Receiver 2e15 also a: conventional amplitude modulation detecting, receiver, similar to receiver 2i. The particular beat frequency determined by the position of. the antenna 2 in. the hyperbolic pattern at. any given. instant isdetected and. amplified by receiver 28 and sent. through an output line 3.! of thereceiver 28 tothe frequency discriminating comparator 3b. The reference. beat frequency is detected andv amplified by, receiver 2? and: similarly sent fromreceiver 2"!- through an outputlinezttotne frequency discriminating. comparator Elli. where the difference in. the above, two) frequcnices at any given instant is compared; The. result of this. comparison. ismadeto appear as a. voltage and current difierencevzhose magnitude and directions depend upon the difference. inv beat. frequencies from. receivers 23" and 2'31 One such frequency. discriminating.comparator suitable for use in. the invention. is shown schematically in Fighlto be described hereinafter. This differoncev may be madeto, register as a visual indication by a suitable ammeter or voltmeter 32 which may be calibrated. to read in terms of deviation from. the selected hyperbolicpath .suchindicated deviation may then be used for navigation of. the.

body with respect to the selected hyperbolic path. The output of. the frequency discriminating comparator may also be sent to a direction correcting servo 33': which controls the stecringm'echanism of the body, thereby providing automatic guidance of the body with respect to the'selected hyperbolic path.

A second embodiment of" this invention is adaptable to navigation and guidance along a scented-hyperbole without" a separatabeat-frequency reference, signal being generated; In this embodiment, shown inxl ig. 4; thereferenc-e beatfrequency generating apparatus shown in Fig: 1 has been removed; and antennas l3. and 14' have additional'lines and 35"11186117EdfWith a' special signal reversing switch arrangement placed in the path of the sweep-signalsfromthe'generator l 6. A" double switch- 39* is-"placcd' in' lines -t'd' and l2 inthemannershown inFi-g. 4'; so that'a switch fill 6; arm 3'! of the double throw switch: 36, by means of terminals 38 and 39 may be made to include or exclude: the signals delaying line 35 with respect to line l2. Another switch arm- 40 of the double throw switch by m'ean's o't a terminal 4|, may send the sweep signals from. generator I i) to: antenna l3 before antenna i i, and, by means oi a terminal E22,. may reverse the si'gnali timing of antennas it and is: by sending the:- sweep signals to antenna is before. antenna 13. The switch arms 3'! and in; being linked, move together to both change the lines" of travel of the signals to change the; length of effective signal. line between antennas [:3 and. l t. Therefore,..by alterthe position of switch 38- in the above manner; the hyperbolic. gridivork pattern, shown Fig. 3, is also'made: to alternate with the beat frequency of only a selected hyperbolic line, remaining unchangcdregardlessi of change in position of the switch. The hyperbole; in the gridwork, which remains unchanged;.is determined b the length of the, line. 35. which? lies between the terminals 353 and 39;

This may be seen as follows. If, for example,

the present embodiment, generator it! is-made to sweep signalsasishownin'higi 2, and these signals are made to pass through line H to antenna it from which they are radiated; and. a short time later reach antenna. l tafrom which they are also radiated, the beat frequency oi a hyperboleh (Fig. 5) will be determined by the-1 signal time di erence'of any point P on. the-hyperbola h from antenna it taken by'two different routes. One route is shown by the line a: and the other is shown by lines y and a.

If the direction of travel of the. signals is re versed so that they are now emitted from antenna it before antenna it. the beat frequency of hyperbole. it willbe the. same'only if thesignal; time delay at point P is kept the same: Since thetime is eropcrtiona-l. tothe distances of signal el, the comparison: may be made. by scalar length quantities. Thus; with the signals trave1- ing from antennas. l3= to It the signaltravel. di tance difference to. hyperbola his For signalstraveling fromantenna it to. l'3-;.the signal travel'distanc-e' difference tohyperbola h is These difierential distance values'maybe made equal by addingtothei smaller a signal-line such as 35. This he expressed where u thelength of the-:line'ti; Therefore;

correspondingly altering'the lengthof line between antennas i3 and lfii with change in switch position selected hyperbole" h willretain a constant beat frequency despite-ohangesinpm sition or switchtt. Unthe'other hand; as-switch changes position; the beat frequencies of hyperbolas oneither side of 'tlie-selcct'ed h h'yperbola will change. For example; when the signals travel from antenna l to t} beat'frequencies to the right of thehyperbola it will progressivel decrease while the" beat frequencies of hyperbolas to the left-of the hyperbola h will progressively increase; when tlie=posi-tionof'switch 3& is changed so that the-signaistravel-froman tenna ld t'o antenna 13* the pattern aboutthe'hyperbola h isreverse'dz The-beat frequencies of hyperbcla's to the right 'of hyperbola 113 new pro 7 gressively increase while beat frequencies of hyperbolas to the left progressively decrease.

In the particular case where the equilateral hyperbola is selected as the desired path, the length of the signal line between antennas l3 and i l will remain the same length for either posi tion of the switch 36. In this instance a double throw switch is not necessary. Switch arm 5'! may be replaced with an unbroken signal line between antennas i3 and M.

An asymmetric time cycling energizer 43 is connected by an arm 43 to the switch 3% so as to cause it to change position in accordance with a cyclic time sharing plan. For example, in this instance, the switch arms as and 3'! are held on terminals ll and 38 for three units of time, and on terminals 42 and 39 for one unit of time. Also 2-3 cycles per second was found to be a convenient time cycle base. However, other cyclic rates may be used equally well. A suitable device for energizing the switch 35 in accordance with such an asymmetric time cycle is shown in Fig. itto be hereinafter described. Any suitable time sharing plan may be used provided it is of an asymmetric nature.

With such a time sharing plan, a left and right sense of direction with respect to the selected hyperbola. h is achieved. For example, to the right of the hyperbola h, a beat frequency below the beat frequency of 71. may be registered for three units of time, and a beat frequency above the heat frequency of h for one unit of time. In such case, to the left of the hyperbola h, the high and low beat frequencies will have an opposite time relation. A sense of degree of deviation from It becomes also apparent because the range between the high and low beat frequencies increases as divergence from it increases.

A suitable receiver apparatus for navigation and guidance in this alternating hyperbolic gridwork is shown in the right-hand side of Fig. A. At a particular point in the gridwork where an antenna 44 is located, two synchronized frequency swept signals S1 and S2 of different instantaneous frequencies radiated from the an tennas i3 and M, respectively, add algebraically, producing a resultant signal S3 whose amplitude varies with time in accordance with the difference in frequency between the two components. The signal S3 is picked up by antenna 44 and sent by line 45 to an amplitude modulation detecting receiver it, similar to receiver 28, where the beat frequency is detected and made to appear in this instance as a signal S4. The signal S; is sent by line 47 to a frequency discriminating circuit which is designed with an output voltage characteristic such that its output depends only on the detected beat frequency. In one such circuit, the signal S4 is sent by line 41 to an amplitude limiter :38 so as to remove any amplitude variations from the signal Si caused, for example, by noise. The amplitude limiter is of conventional design and may be similar to limiters used in existing frequency modulated receivers. A resistance 49 and condenser 4e and rectifier 5D, arranged in lines 5! and 52 from the limiter :8, as shown, effect a voltage response across the load resistance 53 as shown by a curve 55 in Fig. 6. The curve 55 is substantially linear between the points 55 and 55 which correspond to the limits of the spectrum of beat frequencies encountered in operation. A point 56 between points 55 and 55 corresponds to the beat frequency of the selected hyperbolic path it. Two peak rectifiers 51 and 58 are arranged with con- 8. densers 59 and 60, and resistances 6| and 62 of proper proportion to provide a comparator circuit, as shown, in which a current is made to flow through indicator 32 in one or the other direction depending on whether the beat-frequency pattern corresponds to that received by antenna 44 to one side or the other of the selected hyperbola. h. This may be more clearly seen as follows. Because of the asymmetrically varying beat frequencies of a given hyperbolic line as explained above, a typical pattern of voltage across the rectifiers 51 and 58 is shown by a voltage versus time graph in Fig. 7. The direct current components in the comparator circuit are removed by condensers 59 and 60 thereby causing a zero axis 64 disposed so that areas 65 and 66 are equal. Peak rectifiers 51 and 58 measure the peak positive and negative voltages 61 and 68, respectively. The difference between these is taken through resistors 61 and 62 so that a unidirectional current is caused to flow through the indicator 32 and servo 33. Since the spread between voltages 61 and 68 is dependent upon the hyperbolic path on which antenna 44 is located, the magnitude of the above current is a function of the deviation of antenna M from the reference hyperbola.

A frequency discriminating comparator circuit 35 suitable for use in Fig. 1 is shown in Fig. 8.

The best-frequency signal from receiver 21 is sent to an amplitude limiter 69 and the signal from receiver 28 is sent to an amplitude limiter iii. In the limiters, variations in amplitude of the signals are removed, thereby providing a proper base for frequency comparison.

An asymmetric time cycling energizer 'H, for example, as shown in Fig. 10, operates a double throw switch 72 to alternately pass the signals from limiters 69 and it! through lines 13 and 14 to a frequency discriminating and comparing circuit 5, as described above and shown by a dotted line box E5 in Fig. 4. The energizer H has a time sharing plan and a cyclic time base as described with respect to the timing energizer 13 above.

A sweep-frequency generator and antenna arrangement suitable for use in this invention is shown schematically in Fig. 9.

An inductance coil 75 and capacitor H are arranged in an oscillatory parallel circuit across the plate and cathode terminals of a reactance tube 18. A condenser is is inserted between the plate terminal of the tube l8 and one side 19' of the above parallel oscillatory circuit to prevent short circuiting the direct current in the plate circuit of tube 18. The side 19 is also led by a line 88 and grid leak resistance 8! with a bypassing condenser 82 to the grid of a triode 83. The plate circuit of the triode 83 is coupled through a radio frequency choke 85 to the plate circuit of the reactance tube E8 by line 35. A positive potential is maintained across tubes 18 and 83 by connecting to line 85 a power source as battery 85. The cathode of the tube 18 is led to ground while the cathode of the tube 83 is tapped to a suitable point 31 on the inductance coil 15. The plate circuit of a gas triode discharge tube 88 is connected to the positive terminal of the power source 86 by a line 89 having a resistance 95. The plate circuit of the tube 88 is also connected by a line 9| to the grid circuit of the tube l8 through a resistance 92. A condenser 93 connected across the plate and cathode terminals of the gas discharge tube 88 will eifect a linear sweep potential 94 across the condenser 93. this instance, the resistance 9i) andcapacitance 93 are 'a'djustedto produce a sweep frequency of 400 cycles per second; however, other frequencies may be used equally well. This sweep voltage accordingly alters the grid voltage of the 'reac'tance tube 78, which in turn causes the tube 18 to appear as a changing reactance, thereby causing the frequency of oscillation in the parallel capacitance 7'5 and inductance 5 circuit to vary in a similar relation. A secondary coil 95 coupled to the primary coil 76 has induced in it signals of the same frequency which are sent through lines 96 and 97, which may be a coaxial cable, to the doublets 98 and 99 which correspond to the radiators I3 and I4 discussed above.

One example of a suitable time cycling energizer it for switch 35 is shown in Fig. 10. It consists of a multivibrator circuit in which the asymmetry of the time cycle is produced by choosing condensers and resistances of such values that the product of a condenser I80 and resistor Hit in the grid circuit of triode M22 is different from the product of a condenser Hi3 and a resistance I94 in the grid circuit of a triode N35. The extent by which these products differ from each other will determine the proportion of the time sharing plan discussed above. A solenoid IEIE in the plate circuit of the triode M is arranged to activate a lever arm it"? attached by link 33' to the double throw switch 36. A spring I 08 is also attached to the lever arm Iii? to cause it to move to the opposite position when it is released by the solenoid 565. Because of the above difference in products of the capacitances and resistances, the current pattern in the plate circuit of the triode Hi5 will vary nearly as a rectangular wave as shown at I89. This current flowing through the solenoid IIIt causes the solenoid to be energized by each current peak IIG so as to pull the lever arm It? to a stop I01, which may also be the magnetic core of solenoid I06. When the current again reaches a null I I 2, the solenoid Illfi is de-energized, permitting the spring I08 to pull the lever arm ill! back to a stop I I3.

A third embodiment of this invention incorporates some of the features of each of the above two embodiments. The double throw switch 36 energized by the asymmetric time cycling energizer ie is arranged in line I2 between transmitters I3 and It and in a line IM to a transmitter i I in the manner shown in Fig. 11, so that when the line 32 is open, as shown, the transmitter I i5 is connected to an amplitude modulator IIS and, when the line I2 is closed, the transmitter H5 is connected to a sweep modulator I ll. The transmitter i 65 ma be of known conventional design. The amplitude modulator IIS, when combined with the transmitter I I5, forms an amplitude modulated transmitter such as, for example, the amplitude modulated transmitter 29 referred to in Fig. l with an additional means for generating a modulating frequency signal such as, for example, a suitable oscillator. When the sweep modulator IIl is combined with the transmitter i i5, a sweep frequency generator is formed such as, for example, shown in Fig. 9. Thus, with switch arm 46 on a terminal I I8 and switch arm 3? on a terminal IIS, the amplitude modulator causes the transmitter signal to be modulated in amplitude at a frequency equal to the beat frequency of the selected hyperbola h. The signal from transmitter I 5 amplitude modulated at this frequency passes through line II and is radiated by antenna I3 to provide the reference beat frefluency. Because the line I 2 is open, no signal will reach antenna It. I

With the switch arm '46 on terminal I 29, and the switch arm 3? *on terminal 39, the sweep-frequency modulator i '5? causes the signal of transmitter M5 to sweep over a band of frequencies which are sent by line i i to antenna it for radiation and by line i2 to antenna M for radiation a small interval of time later, thereby producing a hyperbolic gridwork of beat frequencies as described above. The asymmetrical time cycling energizer d3 causes switch 35 to alternate in accordance with 'a time sharing plan 'as explained above. Therefore, the gridwork pattern will-a1- ternate with the reference signal, each existing for a number of units of time determined by the cycle of the cycling energizer 43.

A receiving and analyzing apparatus as shown in the right-hand side of Fig. 4 may be used for navigation and guidance in this system as explained above in the operation of that apparatus.

This invention is not limited to the particular details of construction and processes described, as many equivalents will suggest themselves to those skilled in the art. It is accordingly desired that the appended claims be given a broad interpretation commensurate with the scope of the invention within the art.

What is claimed is:

i. In a navagation and guidance system means for producing a fixed frequency signal, means for modulating the amplitude of said signal so as to produce an amplitude modulated signal, means for modulating the frequency of said signal so as to produce a sweep frequency signal, switch means disposed to connect one or the other of said modulating means to said first mentioned means, means for causing said switch means to alternately connect one and the other of said modulating means to said first mentioned means in a continuous asymmetrical time cycle, two means at separated points for radiating signals, a signal link between said first mentioned means and said radiating means disposed to cause the signal of said sweep to radiate from one of said radiators a short time before radiating from the other of said radiators, and switch means in said signal link synchronized with said first mentioned switch means so as to pass said amplitude modulated signal to only one of said radiators.

2. In a navigation and guidance system, first and second signal radiators disposed a fixed distance apart, means providing a signal link between said radiators, means to produce a signal, means connected in circuit with said signal producing means alternately to modulate said signal in one or the other of two difierent fashions in a continuous asymmetrical time cycle, switch means connected between said signal producing means and one of said radiators, said switch means having first and second alternative dispositions, said switch means in said first disposition furnishing said signal to both of said radiators in a first fashion to produce a first radiation pattern, and in said second disposition furnishing said signal to one only of said radiators in a second fashion to produce a second radiation pattern, and means for causing said switch means alternately to assume one or the other of said dispositions in said continuous asymmetrical time cycle.

3. In a navigation and guidance system, first and second signal radiators disposed a fixed distance apart, means providing a signal link between said radiators, means to produce a sweep frequency signal and a fixed-frequency signal, switch means connected between said signal producing means and said radiators, said switch means having first and second alternative dispositions, said switch means in said first disposition furnishing said sweep frequency signal to said first radiator and through said link subsequently to said second radiator, said switch means in said second disposition furnishing said fixed frequency signal to only said first radiator, and means for causing said switch means alternately to assume one or the other of said dispositions in a continuous asymmetrical time cycle.

References Cited in the file of this patent UNITED STATES PATENTS Number Name Date Brunner Nov. 23, 1948 Englund July 21, 1931 Evans et a1. Oct. 31, 1933 Honore Feb. 21, 1939 Guanella Dec. 31, 1946 Dingley Jan. '7, 1947 Fletcher July 1, 1947 Guanella Oct. 19, 1948 

